The subject invention is directed to sheet products having unique configuration, their use as separators in batteries and to an improved method of forming said sheet product.
Storage batteries have at least one pair of electrodes of opposite polarity and, generally, have a series of adjacent electrodes of alternating polarity. The current flow between the electrodes is maintained by an electrolyte which can be acidic, alkaline or substantially neutral depending on the nature of the battery system. Separators are located in the batteries between adjacent electrodes of opposite polarity to prevent direct contact between the oppositely charged electrode plates. It is highly desired to have the separator (i) be thin and light weight to aid in providing a battery of high energy density; (ii) have a structure that inhibits dendrite formation between the electrode plates; (iii) have the ability to enhance the uptake and cause substantial uniform distribution of the electrolytic composition over the electrode plates (generally referred to as wicking); and (iv) provide the properties of freely permitting electrolytic conduction. It is further highly desired to be able to produce the separator sheet product in an economical and environmentally safe manner while achieving sheet product free of defects, such as pinholes and the like.
Separators conventionally used in battery systems are formed of polymeric films which, when placed in an electrolyte or electrolytic system, are capable of exhibiting a high degree of conductivity while being stable to the environment presented by the battery system. The film may be macroporous such as those formed by spun glass and the like. Alternately, the film may be microporous, such as those formed from polymeric films having fillers distributed throughout the film. The fillers aid in the adsorption of electrolyte and provide wicking properties to the sheet product. Such sheets are disclosed in U.S. Pat. Nos. 3,351,495 and 4,287,276.
Microporous sheet products have also been formed from polymeric compositions having a liquid plasticizer which, when removed by extraction methods, provide the resultant sheet with its microporous structure. Conventionally, such plasticizers are of high molecular weight oils and the like with the idea of providing compatible properties with respect to the polymeric material during the initial steps of the sheet's formation while being incompatible and readily extractable during process formation. Extraction is conventionally done by washing the plasticizer from the cooled initially formed sheet using a compatible, low molecular weight second liquid. The voids resulting from the removal of the plasticizer provide substantially uniform porosity throughout the resultant separator sheet product. The resultant mixed liquid is a waste by-product of the described process.
A still further mode of forming microporous sheet product is by stretching and annealing polyolefin sheet material to cause microporosity in the treated sheet. Such processes are disclosed in U.S. Pat. Nos. 3,426,754; 3,558,764; 3,679,538; 3,679,540; 3,801,404; 3,843,761; 4,138,459; 4,994,335; and 5,328,760. For example, a stretched microporous polypropylene is described in U.S. Pat. No. 3,679,540. The polymer is extruded under low melt temperatures with high shear stress in order to maximize polymer molecular alignment within the extruded film. The film is then annealed to consolidate the crystalline polymer network followed by stretching at low temperatures to cause tearing to occur in the crystalline regions from remaining amorphous regions. Micro-cracks are thus formed. The film is further stretched and annealed in sequential manner to cause the cracks to form into pores. The porosity of the resultant film is limited to less than about 40% and is not well controlled.
U.S. Pat. No. 4,247,498 discloses a process for forming a microporous polymeric sheet product by blending a mixture of a polymer and a compatible liquid into a homogenous mixture. The mixture is cooled under non-equilibrium conditions to initiate liquid-liquid phase separation causing droplets of the liquid to form which are each surrounded by liquid polymer. Further cooling is required to cause the polymer to solidify and then the liquid is extracted using a second liquid to produce a structure having a series of enclosed cells of substantially spherical shape which are interconnected to adjacent spherical cells. This process must be conducted in a time consuming manner and requires extraction of the initially compatible liquid by other liquids which generates a waste by-product stream of the combined liquids.
U.S. Pat. No. 4,539,256 is also directed to the formation of sheet product suitable as a microporous battery separator. The reference discloses the use of liquids having solubility and hydrogen bonding parameters which are within a few units of the respective values of the crystallizable polymer. High molecular weight mineral oil, higher ester phthalates and the like are taught to be suitable for forming initial mixtures with polyolefins. The liquid used in the initially formed polymeric sheet product must then be removed by extraction. The reference teaches that hexane or alcohols are suitable for extraction of the above described liquids. The resultant extraction mixture is a waste product that requires special handling. The resultant sheet product is described as having randomly dispersed spaces and thermoplastic plastic particles connected to each other by a plurality of polymeric fibrils. The fiber/particle arrangement causes the resultant porous product to exhibit poor tensile and puncture strengths.
U.S. Pat. No. 4,948,544 discloses a method of forming a film product by initially forming a film from a solution of a polyolefin and a first solvent such as hydrocarbon or decalin; bringing one surface of the film into contact with a second solvent; passing the film through a cooling bath containing a cooling agent selected from gas, water or hydrocarbon liquids; removing the solvents from the film at a temperature below the dissolution temperature; and stretching the film in one or more directions in the plane of the film. The final article is substantially non-pores and formed in a manner that produces a waste stream composed of a mixture of solvents.
U.S. Pat. No. 5,051,183 discloses the formation of a porous article with ultrahigh molecular weight polyethylene. The rate of cooling is taught to have an effect on the degree of crystallization.
U.S. Pat. No. 5,503,791 discloses a method of causing phase-separation within a film by contacting both sides of the film with a second solvent before the film is contacted with a cooling agent (e.g. water). The reference teaches that the density of the second solvent must be smaller than that of the cooling agent.
U.S. Pat. No. 5,830,554 disclosed a process for manufacturing a microporous polyolefin film which can be used for various battery separators. The process incorporates a polyolefin polymer (Component I) with a first diluent (Component II) and a second diluent (Component III) to form a thermodynamic single phase which can undergo thermodynamic liquid-liquid phase separation. The article is then subjected to extraction with methylene chloride to produce porosity followed by rapid cooling with cold air or cold water. The resultant mixture of liquids produces a hard to handle waste stream.
U.S. Pat. No. 7,479,243 discloses a method of producing a microporous polyolefin membrane comprising the steps of extruding a solution of a polyolefin and a solvent to produce a gel-like article, cooling the article by direct contact with cooling air, cooling water, other cooling media, or a cooling roll. It further disclose a method of treating the newly formed membrane while still in the gel-like form by first removing the pore-forming solvent originally within the gel-like form followed by treating the gel-like article with a hot solvent, such as liquid paraffin.
U.S. Pat. No. 7,815,825B2 disclose a method of treating the newly formed membrane while still in the gel-like article form with an organic solvent maintained at a temperature of 110 to 130° C.
WO 2007/117042 discloses a method of forming a microporous membrane composed of polyethylene having at least one percent of ultra-high molecular weight polyethylene. The membrane is formed by melt blending the polymers with a liquid polymer solvent, extruding and cooling the melt to produce a gel-like sheet. The sheet is then stretched and the stretched sheet is washed with a liquid capable of displacing the polymer solvent to form a porous membrane. The resultant porous membrane is stretched and heat set under the same temperature conditions to provide the resultant microporous membrane. The process includes known steps of stretching the initial sheet while retaining the polymer solvent in the sheet. The solvent is then removed by common extraction process using a second liquid to result in a by-product composed of a mixture of polymer solvent and extraction liquid. Finally, the liquid free sheet is further stretched to form their microporous membrane.
None of the above references directs one to the usage of the same high volatile, low molecular weight organic fluid for forming an initial mixture with a polyolefin and as the cooling media, to process the initially shaped polymer composition under certain sequential temperature and stretching process conditions while concurrently causing vaporization of the fluid to produce the unique sheet product structure, as described herein below.
The properties of sheet products useful as battery separators include not only permeability, mechanical strength, and dimensional stability, but also properties related to electrolytic solution absorption, and battery cyclability. It is highly desired to achieve a thin, light weight sheet product that has high wicking capability which provides the battery with the ability of maintaining electrolyte over the electrode surfaces and achieving high electrolytic conductivity while, when appropriate, providing high inhibition to formation and growth of dendrites between electrode elements of opposite polarity. Further, electrodes for lithium ion batteries are known to undergo expansion and contraction according to the intrusion and departure of lithium. Because separators are compressed when the electrodes expand, it is desirable that the separators have the ability to undergo such compression while exhibiting, when compressed, as little a decrease as possible in electrolytic solution retention. Further, it is desired to achieve a sheet product having the above properties in a cost effective and environmentally desired manufacturing manner as by utilizing the processing material in cyclical manners and by producing little or no waste stream.